The long term solution to this is MDL-76231, but we might want to find a fix for this is stables.

Steps to reproduce:

1. This is really a problem in Chrome. Behaviour in Firefox seems better.
2. Go to anywhere that does Ajax requests, e.g. a filepicker. (E.g. https://qa.moodledemo.net/mod/assign/view.php?id=5&action=editsubmission logged in as student.)
3. Click on the file that is already there to open the popup.
4. Disconnect your internet.
5. Open browser developer tools, and switch to the Network tab, so you can see what is going on.
6. Alter the filename and click Update.
7. Dialogue changes to show a progress spinner ...
8. ... and just sits there.
9. In developer tools, you can see the AJAX request already failed, but this has not been reported to the user nicely. This short be reported like any other JavaScript error.